Monitoring time-dependent macromolecular dynamics in a crystal can elucidate structure-function relationships and thus shed light on how protein molecules really work. Synchrotron sources have advanced to the point that important macromolecular dynamic experiments are currently being performed but the detectors are lacking. The proposed Time-Resolved X-Ray Detector (TRXD) is an electronic time- resolved two-dimensional pixel-array detector that should make a very important contribution to the field of time-resolved crystallography. Mechanical gating and image-plate exchanging, typical of current two- dimensional pixel-array time-resolved x-ray imaging systems, are replaced by two dimensional solid-state detector arrays and electronic switching and storage. Time resolutions in the nanoseconds (more than seven orders of magnitude faster than a mechanical image-plate exchanger) or as low as 10 ms are possible. The Phase Il detector will have the capability to record, 15 consecutive, two-dimensional diffraction-pattern time integrations with a spatial resolution of 150 mu m; move-camera-like frames can be recorded at a speed of better than lOO MHz. In one or two years, the available technology should support 25 time integrations at a spatial resolution of 81 mu m, or lOO time integrations at a spatial resolution of 120 mu m; the highest time resolution will probably be 1 ns. PROPOSED COMMERCIAL APPLICATIONS: The TRXD can be also be used for time-resolved x-ray or visible-light applications in addition to crystallography. It is a two-spatial- dimensions, high-time-resolution replacement for conventional X-ray cameras or with modification, visible-light streak cameras.